Technical Field
An improved internal gear pump is disclosed. More specifically, one disclosed internal gear pump includes a controller linked to a stepper motor for enhanced dispensing accuracy. Still another disclosed internal gear pump includes an improved head design for enhanced accuracy. Further, algorithms for providing precise pump control and dispensing accuracy are also disclosed.
Internal gear pumps are known and have long been used for the pumping of thin liquids at relatively high speeds. The typical internal gear pump design includes a rotor mounted to a drive shaft. The rotor includes a plurality of circumferentially disposed and spaced apart rotor teeth that extend axially toward an open end of the pump casing. The open end of the pump casing is typically covered by a head plate or cover plate which, in turn, is connected to an idler. The idler is mounted to the head plate eccentrically with respect to the rotor teeth. The idler also includes a plurality of spaced apart idler teeth disposed between alternating idler roots. The idler teeth are tapered as they extend radially outward and each idler tooth is received between two adjacent rotor teeth. The rotor teeth, in contrast, are tapered as they extend radially inward. A crescent or sealing wall is disposed below the idler and within the rotor teeth. The crescent provides a seal to prevent the loss of fluid disposed between the idler teeth as the idler teeth rotate. The rotor teeth extend below the crescent before rotating around to receive an idler tooth between two adjacent rotor teeth.
The input and output ports for internal gear pumps are disposed on opposing sides of the rotor. The fluid being pumped is primarily carried from the input port to the output port to the space or roots disposed between adjacent idler teeth. This space may be loaded in two ways: radially and axially. The space is loaded radially when fluid passes between adjacent rotor teeth before being received in a root disposed between adjacent idler teeth. Further, there is typically a gap between the distal ends of the rotor teeth and the head plate or casing cover which permits migration of fluid from the inlet port to an area disposed between the head plate and the idler. After migrating into this area, the fluid can be sucked into the area or root disposed between adjacent idler teeth during rotation of the idler and rotor.
In order to increase the speed of such internal gear pumps, head designs have been developed to ensure complete loading of the inner most area between the idler teeth or the root disposed between the adjacent idler teeth. One such design is disclosed in U.S. Pat. No. 6,149,415.
However, while the head design disclosed in the ""415 patent and other internal gear pumps known in the art have increased the pumping rate of such internal gear pumps, such designs have been found unsatisfactory for applications where precise dispensing of relatively small amounts of liquids is required.
Accordingly, there is a need for an improved internal gear pump design with improved accuracy.
Several embodiments of improved internal gear pumps and pumping systems are disclosed which satisfy the aforenoted need.
Specifically, an internal gear pump is disclosed which includes a stepper motor coupled to a drive shaft that, in turn, is coupled to a rotor. The rotor is meshed with an idler which, in turn, is mounted to a head coupled to a head plate. The improvement comprises a controller linked to the stepper motor. The stepper motor imparts a stepped rotational movement to the drive shaft wherein a single 360xc2x0 rotation of the drive shaft comprises a plurality of steps. The controller sends a signal to the stepper motor to rotate the drive shaft a predetermined number of steps. The signal causes the stepper motor to rotate the drive shaft the predetermined number of steps. The controller calculates the predetermined number of steps based upon a dispensed amount that is inputted to the controller. The controller calculates the predetermined number of steps and generates the signal sent to the stepper motor based upon an algorithm derived experimentally that defines a relationship between dispense amount and a number of steps required for each dispense amount that is unique to each fluid to be pumped.
Typically, the relationship between dispense amount and the number of steps required is a linear relationship that can be defined experimentally with a plurality of data points for a particular liquid. A straight forward algorithm is generated for the liquid to be pumped and stored in the controller memory.
Instead of, or in addition to, the above-described controller system, an improved head design is also disclosed. In the improved head design, the head comprises a head surface that faces towards the rotor. The head surface consists of an aperture for receiving the idler pin, a crescent disposed below the aperture and a remaining planar head surface area that surrounds the aperture and the crescent and that abuttingly engages the rotor and idler. The idler pin extends outward from the aperture in the head surface and the idler comprises a central hole that mateably receives the idler pin so that the idler abuttingly engages a first circular ring area of the head surface disposed above the crescent and around the central aperture. The rotor abuttingly engages a second circular ring area of the head surface area that extends below the crescent and partially overlaps the first circular ring area. The first and second circular ring areas are eccentric with respect to each other and account for the planar head surface area. The terms xe2x80x9cabovexe2x80x9d and xe2x80x9cbelowxe2x80x9d are used in a relative sense. In some embodiments, the pump may be arranged where the crescent is disposed vertically above the aperture which accommodates the idler pin. Thus, the first circular ring area extends around the aperture and between the aperture and the crescent. The second circular ring area extends around the crescent wherein the crescent is disposed between the portion of the second circular ring area and the aperture.
In a further refinement, the head and head plate comprises a two-piece assembly wherein a wave spring is disposed between the head and the head plate and the wave spring biases the head towards the rotor.
In another refinement, the head and head plate are unitary in construction.
In a further refinement, the stepper motor is frictionally coupled to the drive shaft which, in turn, is frictionally coupled to the rotor. In a further refinement of this concept, the stepper motor is press fitted to the drive shaft which, in turn, is press fitted to the rotor.
In a further refinement relating to the embodiment including a controller, the controller is linked to a power supply which, in turn, is linked to the stepper motor. The above-described signal is sent from the controller to the power supply which transmits sufficient power to the stepper motor to rotate the drive shaft a predetermined number of steps corresponding to the signal.
In another refinement, each of the above-described steps corresponds to approximately 1.8xc2x0 of rotation of the drive shaft so that one rotation of the drive shaft is approximately equivalent to 200 steps. In a further refinement, half-steps are available where each half-step corresponds approximately to 0.9xc2x0 of rotation of the drive shaft so what one rotation of the drive shaft is approximately equal to 400 half-steps. Generally speaking, in depending upon the stepper motor selected, the steps can correspond to a rotation of the drive shaft ranging from about 0.5xc2x0 to about 3xc2x0 so that one rotation of the drive shaft can range from about 720 to about 120 steps.
In another refinement, instead of operating based upon an open loop utilizing an algorithm as described above, the controller can operate based upon a closed loop. In such a refinement, the controller is linked either directly or indirectly to an output mechanism which may be in the form of a scale that weighs the fluid being pumped or dispensed from the pump, a fluid level indicator in a receptacle that measures the volume of fluid being pumped or a pressure transducer that measures the pressure or flow rate of the fluid being pumped. The output mechanism generates an output signal which is communicated to the controller. Initially, the controller sends a dispense signal to the stepper motor to rotate the drive shaft. The dispense signal causes the stepper motor to rotate the drive shaft. The controller generates a stop signal and sends a stop signal to the stepper motor based upon an output signal received from the output mechanism that indicates that the dispense amount has been reached.
In yet another refinement, a method for controlling an internal gear pump is disclosed. The method comprises linking a controller to the stepper motor, the controller comprising a memory, deriving an algorithm experimentally that defines a relationship between dispense amount and the number of steps that is unique for each fluid to be pumped, storing the algorithm and the memory of the controller, communicating a dispense amount to the controller, calculating the number of steps in the controller for dispensing the dispense amount using the algorithm and sending a signal from the controller to the stepper motor to rotate the drive shaft the calculated number of steps.
Other features and advantages of the disclosed internal gear pumps, control systems therefore and methods of controlling an internal gear pump will be apparent from the following detailed description and appended claims, and upon reference to the accompanying drawings.